This primary focus of this project is to evaluate the pathophysiological mechanisms of thrombocytopenia induced by an infusion of liposome encapsulated hemoglobin (LEH) and to quantitate changes in oxygen uptake and oxygen delivery of LEH over time after administration in small animal models. We plan to investigate this LEH-induced thrombocytopenia using both in vitro and in vivo radiolabeled platelet studies in rat and rabbit models. The proposed in vivo studies will use non-invasive imaging of platelets labeled with indium- 111 to assess the effect of complement depletion, inhibition of arachidonic acid metabolism, and in vivo platelet aggregation inhibitors on platelet biodistribution following administration of LEH. Formulations of LEH less likely to cause thrombocytopenia will be investigated. A second aim of this project is the investigation of LEH oxygen uptake and tissue delivery. Specifically, several new techniques based on the short lived positron emitting radioisotope of oxygen, oxygen-15 (15-O) (t 1/2 = 2.O2 minutes), will be used to investigate the ability of LEH to load oxygen from the lungs and deliver oxygen to the tissues in small animal models (rats and rabbits). Changes in oxygen carrying capacity and oxygen tissue extraction will be assessed over a period of time post-infusion and correlated with the clearance time course. Formulations of LEH that may potentially have prolonged circulation times, a longer effective oxygen carrying capacity, and efficient oxygen delivery to the tissues will be assessed using these new techniques. These studies will provide important information necessary for the development and continual improvement in the formulation of LEH as a safe and efficacious blood substitute. These studies will provide a better understanding of both the mechanisms of toxicity and the factors which govern the delivery of oxygen to the tissues. The tracer techniques described in this proposal will provide new tools that could generally be applied to the study of blood substitutes.